The present invention relates generally to distance measuring devices and deals more specifically with a distance measuring device arranged as a relaxation oscillator wherein the frequency of oscillation is a measure of the distance between the device and a remote target.
Distance measuring devices using the concept of transmitting an electromagnetic wave signal toward a target and then measuring the time lapse to receive a reflected wave as a measurement of the distance between the device and the target are generally known in the art. A distance measuring device of the time lapse type is described in U.S. Pat. No. 3,246,322 issued Apr. 12, 1966 to Kuecken. Kuecken discloses the transmission of finite duration continuous wave signals modulated at a given frequency wherein reflection of the signal is used to control the modulation rate of the transmitted signal. The transmitted signals are generally of short duration and the reception of a reflected signal is used to activate the transmitter to generate a new signal. The time interval between output signal generations is the round-trip travel time of the transmitted signal from the device to a reflecting target and back to the device.
Another distance measuring system of the time lapse type is disclosed in U.S. Pat. No. 3,503,680 issued Mar. 31, 1970 to Schenkerman which uses a concept similar to Kuecken in that an echo return pulse is used to activate a transmitter to send out another pulse wherein the process is repeated until a predetermined number of echo pulses have been received. The time required to receive the predetermined number of pulses is proportional to the distance.
There are a number of disadvantages associated with the known distance measuring devices which have an adverse effect on the accuracy of a measurement. Devices in which time is measured to determine distance are generally effected by low signal-to-noise ratios.
Additionally, the known devices often employ numerical averaging techniques to average a relatively large number of time interval measurements to increase the accuracy of the time measurement used in determining the distance.
Another disadvantage associated with the known measuring devices is the large systematic measurement errors that may occur due in part to the high number of round-trip propagation times that are detected in a predetermined time interval. Other errors are in part due to the less than precise pulse detection techniques used to sense the start and end of a reflected signal wherein such techniques include for example, detection of the zero crossing of a reflected signal. The sources of error are applicable not only to continuous wave type distance measuring devices but also to those in which pulse signal techniques are used.
A further disadvantage of the known distance measuring devices is the relatively high cost and expense associated with the complex electrical components and subassemblies required to transmit and sense reflected pulses and to time average propagation times.
It is an object of the present invention therefore to provide a distance measuring device that overcomes the drawbacks and disadvantages associated with known distance measuring devices.
It is a further object of the present invention to provide a distance measuring device configured as a relaxation oscillator wherein the oscillation frequency is used to determine the distance between the device and a remote target.
It is yet a further object of the present invention to provide a distance measuring device that is constructed of relatively low cost components and yet achieves highly accurate distance measurements.